1. Field of the Invention
This invention relates to solar simulators in general, and more particularly to a solar simulator having an accurate output spectrum of extreme uniformity and that is easily adjusted to produce a variety of desired solar spectra.
2. Brief Description of the Prior Art
Solar simulators are light source devices for reproducing the spectral distribution of natural sunlight As such, solar simulators have proven indispensable in evaluating various devices using solar energy, such as, for example, the photoelectric conversion properties of solar cells. Typically, the solar cell being evaluated or tested is illuminated by the output light beam produced by the solar simulator. The voltage and current parameters of the cell as well as the overall conversion efficiency of the cell are then measured and determined Since solar cells are designed for a number of different applications, that is, some cells are designed to operate in space while others are designed to operate on the surface of the earth, there is a need to evaluate the performance of each type of cell using the same solar spectrum in which it is designed to operate. For example, the spectral distribution of light from the sun in space is not the same as the spectral distribution after the light has passed through the earth's atmosphere. More specifically, sunlight in space contains more infrared and ultraviolet radiation than does sunlight incident on the surface of the earth. Accordingly, several standardized solar spectra have been developed for sunlight in various places and under various conditions. The standard reference spectrum of sunlight in space is usually referred to as the AM0 spectrum. Similarly, the AM1.5 spectrum identifies the solar spectrum on an average sunny day while the AM2 spectrum designates that solar or sun spectrum found on the surface of the earth at sea level. Therefore, to accurately test solar cells designed for use in space requires a solar simulator that can accurately reproduce the AM0 spectrum. Cells designed to work on earth should be tested using the AM1.5 or AM2 spectra.
Several different systems have been used in the past to produce output beams that approximate these various solar spectra. Principal among them are single and dual source solar simulators. The single source simulators generate their light outputs from the light produced by a single light source. Dual, or twin source models, on the other hand, utilize the light produced by two light sources to approximate the desired solar spectrum. Designers of systems employing both technologies are continually attempting to improve the simulators. Performance issues of particular importance include accuracy of the reproduced spectrum as compared to a standard spectrum (i.e., either AM0, AM1.5, or AM2), light output or intensity (brightness) capability, good beam uniformity, overall light efficiency, and size. Ease of manufacture and cost are also important considerations.
The single source simulators produce output beams from single light sources only, typically xenon arc lamps. High brightnesses and beam uniformities are usually easily obtained with such single source systems, but it is difficult to accurately reproduce a given standard solar spectrum, such as AM0, AM1.5, or AM2, because the light produced by the xenon arc lamps contains fairly strong intensity peaks in the near infrared region that are not found in any of the standard solar spectra. Although improvements have been made in the infrared portion of the spectra produced by such simulators by using various types of filters, there are still fundamental limits to the accuracy of the spectral distribution produced using only one source. Another drawback associated with such systems is that it is difficult to modify the spectral distributions of the output beams to reproduce one of the other standardized spectra. For example, it is relatively difficult to modify an AM0 simulator to accurately reproduce the AM1.5 spectrum. Usually, such modification requires numerous filter changes, and may possibly require that the light source be driven at a different intensity to reproduce the desired solar spectrum. Such single source simulators were, however, usually sufficient for evaluating the older, first generation of single junction solar cells.
Unfortunately, recent advances in solar cell technology and the development of highly efficient multijunction (MJ) solar cells are placing increasing demands on the testing procedures used to evaluate solar cells. For example, it is now well recognized that the current and voltage characteristics of an illuminated solar cell are strongly influenced by the spectral distribution of the light beam produced by the solar simulator. In fact, the measured efficiency of a typical advanced multijunction (MJ) solar cell can vary by as much as 20% when illuminated by supposedly identical solar simulators. This large variation in measured cell efficiency is due to the spectral mismatch between the simulators, even though each is supposed to accurately reproduce the same standardized spectrum. Obviously, such large variations in measured cell performance hinders further development and improvement of solar cells, since it is difficult to determine which cell performance data best represent the true performance of the cell. In short, it has been found that currently available single source solar simulators cannot adequately match the standardized solar spectra required for advanced MJ cell testing regardless of the numbers and complexities of the filters added to the simulators. Put in other words, while such single source solar simulators were generally acceptable for testing single junction solar cells, they have proven almost completely useless in testing the new advanced and highly efficient multijunction (MJ) cell designs.
The so-called dual source simulators were developed in attempts to minimize the performance measurement errors caused by the spectral mismatch found in single source simulators. The dual source solar simulators combine the light beams produced by two different sources and have shown some promise in better approximating the spectral distribution of the particular solar spectrum that they are designed to reproduce. Typically, such dual source systems still utilize a xenon arc lamp to produce the visible and ultraviolet portions of the spectrum. However, the dual source simulators add a tungsten filament lamp to produce the infrared portion of the solar spectrum. Accordingly, the dual source systems have provided some improvement in the suppression of the near infrared emission lines in the output light by replacing the undesirable infrared light from the xenon lamp with the infrared light produced by the tungsten source, which has a much better spectral distribution in the near infrared region. Unfortunately, while the dual source systems usually provide better spectral accuracy, they usually suffer from output beam nonuniformity as well as reduced efficiency. Also, since two light sources are used, there is usually some change or drift in the spectral distribution of the output beam as the two individual sources age and their output spectra change. Like the single source systems, it is usually difficult to modify a given dual source system to reproduce one of the other standardized spectra. Also, it is even more difficult to vary the intensity of the output beam of a typical dual source system than for a single source system, because the intensities of both light sources must be changed. Obviously, changing the intensity of a tungsten source usually results in a change in the output spectrum from that source, therefore requiring additional filters to bring the spectral distribution of the output beam back to its original configuration.
A fairly recent dual source solar simulator is disclosed in the Kusuhara patent, U.S. Pat. No. 4,641,227, issued on Feb. 3, 1987. Essentially, Kusuhara's simulator includes a dichroic beam splitter to remove the near infrared component from the spectrum of the light produced by the xenon arc lamp while simultaneously extracting the near infrared component from the spectrum of the light produced by the tungsten filament lamp and combining it with the light emitted by the xenon arc lamp minus the previously filtered near infrared component. An optical integrator integrates the spectra from the two beams and projects them as a single beam onto the device being tested. Kusuhara's system also includes relatively complex apparatus to vary the intensity of the output beam by changing the distance of the tungsten source from the dichroic mirror and by controlling the electric current supplied to the xenon arc lamp. Thus, by changing the position of the tungsten source, as opposed to changing its intensity, Kusuhara avoids changing the spectral distribution of the light produced by the tungsten lamp, thereby eliminating the need to change the filters. The position of the tungsten source and the current supplied to the xenon arc lamp are synchronously controlled thereby keeping the ratio between the intensities of the light beams from the two light sources substantially equal as the intensity is varied.
While the system disclosed by Kusuhara represents an improvement over the prior dual and single source solar simulators, it still has some disadvantages. For instance, the relatively large dichroic beam splitter used to filter out the near infrared portion from the xenon beam and combine the near infrared beam from the tungsten filament source is expensive and may suffer discontinuities over its surface, thereby causing nonuniformities in the output beam. Further, while the spectrum produced by Kusuhara's twin source simulator is better than that obtainable with single source systems, current research indicates that the use of two sources still does not provide sufficient flexibility to reproduce a desired solar spectrum with an accuracy necessary to provide accurate performance measurements on advanced solar cell designs. Kusuhara's system does not readily lend itself to the addition of additional light sources to better approximate the desired solar spectrum, nor is it easy to change the spectral distribution of the output beam to reproduce one of the other standardized spectra. Also, the complex intensity control apparatus increases the overall cost of the system and introduces yet more complexity. Finally, the power transmission efficiency of Kusuhara system is relatively low, with a significant amount of power being absorbed by the relatively large optical elements and dichroic beam splitter, thus possibly requiring the use of external cooling systems when used at high power levels.
Since these dual source systems still are not capable of accurately reproducing a desired solar spectrum, the testing of the new, multijunction solar cells is still very difficult and tedious. For example, a commonly used testing method for single or dual source simulators involves adjusting the solar simulator until the short circuit current of a known solar cell (i.e., a reference cell) is equal to its calibrated value for a given set of reference conditions, such as temperature, total irradiance, and spectral irradiance. Usually, the reference cell chosen should have a spectral response very similar to the expected response of the new cell being tested to reduce spectral mismatch errors. Unfortunately, reference cells do not exist for many of the advanced multijunction photovoltaic devices currently being developed and tested. Accordingly, errors are introduced into the performance figures by spectral mismatch that cannot always be corrected. For example, for two terminal multijunction devices, performance parameters such as fill factor and efficiency are closely related to the spectral content of the light incident on the cell. Even after making corrections for spectral mismatch, the accuracy in the measurements made with the current simulators is limited to about 5% for single junction cells and as high as 20% for multijunction cells.
Therefore, there remains a need for a solar simulator that can accurately reproduce a desired solar spectrum at a number of different brightness levels while maintaining a constant spectral distribution and good beam uniformity. Ideally, the spectral distribution of the output light beam of such a system should be easily adjustable to reproduce any of the three standardized solar spectra. Such a simulator should also be very efficient, low power consuming, and relatively inexpensive and easy to manufacture. Further, such a system should contain a minimum number of large, difficult to manufacture, and expensive components, so as to minimize changes in the spectral distribution of the output beam as the components age. Such a system should also minimize the use of expensive quartz elements necessary to transmit the ultraviolet wavelengths of the reproduced spectrum. The solar simulator should be able to combine the light generated by three or even more sources to even more accurately reproduce the desired spectral distribution. Prior to this invention, no such solar simulators existed.